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Old 15th October 2011, 08:11 PM   #11
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Here's a link from Elliott Sound Products on how to measure it; it's quite easy. You'll need a PC based sine wave generator (I'm sure that you can find free software) if you don't have a function generator. The function generator will come in handy when testing an amp if you modify or build one also.

If you are testing with a power amp (no volume control) make sure that you have a means of attenuating the signal so you don't blow your head off (or your speakers).

Measuring Loudspeaker Driver Parameters

Tube amps like a well behaved impedance plot, not too many dips & peaks from 20-20Khz, the flatter the better. There will be an impedance peak at driver/system resonance.
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Old 16th October 2011, 12:23 AM   #12
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Unfortunately, a full plot is needed to find where the minimum impedance lies. Most likely, it will be in the lower midrange or bass regions.

Paul Joppa has provided us with a useful rule of thumb. Joppa's rule states, for a "typical" listening space, an amp speaker combo should be capable of producing 102 dB. SPL peaks at a 1 M. distance.

It's beginning to look like 30 WPC, minimum, is what's needed. Mullard style circuitry using Soviet surplus 6П3С-Е (6p3s-e) O/P tubes or an "El Cheapo Grande" (7591 "finals") are possibilities. Acquiring suitable magnetics, without busting the $450 or thereabouts budget, is going to be key. For example, the well thought of English firm of Sowter charges 173.67 each for their suitable model UA21 O/P transformer. A pair of those would leave little for all the other parts.

Building a decent amp within budget is easy enough. Building a decent amp that mates with the KEF speakers and not spending too much is a challenge.
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